A large number of electrophotographic methods have been conventionally known. A general electrophotographic method involves: utilizing a photoconductive substance; forming an electrostatic latent image on a photosensitive member by using various means; developing the latent image with toner to provide a visible image; transferring the toner onto a recording material (transfer material) such as paper as required; and fixing the toner image to the recording material by using heat or pressure to provide a copied article. The toner remaining on the photosensitive member without being transferred is cleaned by means of various methods, and then the above steps are repeated.
In an electrophotographic method, the step of developing an electrostatic latent image is intended for forming an image on the electrostatic latent image by utilizing an electrostatic interaction between a charged toner particle and the electrostatic latent image. Examples of a method of developing an electrostatic latent image by using toner are as follows: a magnetic mono component development method involving the use of magnetic toner obtained by dispersing a magnetic material into a resin; a non-magnetic mono component development method in which non-magnetic toner is charged by using a charge-providing member such as an elastic blade so that the toner can be used for development; and a two component development method involving mixing non-magnetic toner with a magnetic carrier; and so on.
At present, a technique for exposing a photosensitive member by using a laser beam having a small diameter and the like has developed, and an electrostatic latent image has become fine. An electrostatic latent image may be faithfully developed and improved image quality may be outputted, therefore, reductions in diameters of toner particles and carrier particles have progressed in any one of the above development methods. In particular, attempts have been frequently made to reduce the average particle diameter of toner to improve image quality.
A reduction in average particle diameter of toner is an effective way to improve image quality properties, in particular, granularity and character reproducibility. However, the reduction still involves problems to be alleviated in terms of specific image quality items, in particular, fogging at the time of durable printing, fusion to a photosensitive member, toner scattering, and the like.
A possible cause for the problems is a reduction in charge amount of toner due to the following two facts: the fact that an external additive added to a toner particle deteriorates owing to the long-term use of toner and the fact that a charge-providing member such as a sleeve or a carrier and a regulating member for keeping the coating property of toner on a sleeve at a predetermined value are contaminated by the toner or the external additive, that is, spent occurs. The phenomenon is apt to occur when the size of toner is reduced. To be specific, triboelectric charging is performed by a physical external force such as contact or collision between toner and a sleeve in a mono component developer or between toner and a carrier in a two component developer. As a result, none of the toner, a charge-providing member (such as a sleeve or a carrier), and a regulating member can be prevented from being damaged. For example, in the case of the toner, an external additive added to the surface of the toner may be embedded in the toner, or a toner component may fall out. In the case of the charge-providing member or the regulating member, such members may be contaminated by a toner component containing an external additive, or a coat component with which the charge-providing member is coated so that charging is properly stabilized may wear or be destroyed. Furthermore, a photosensitive member or a charging member for the photosensitive member may be contaminated by an external additive liberated from the toner. As the number of printing increases, those damages preclude the maintenance of the initial properties of a developer, and become causes of fogging, contamination in a machine, and fluctuation in image density.
By the way, the incorporation of a release agent into a toner particle has been disclosed in a large number of documents (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).
A release agent is used for improving offset resistance upon low-temperature fixation or high-temperature fixation of toner, or for improving fixing ability upon low-temperature fixation. On the other hand, the release agent reduces the blocking resistance of toner or reduces the developing ability of the toner owing to a temperature increase in a developing device. In addition, when toner is left for a long time period, a release agent exudes to the surface of toner particle, so developing ability may be reduced.
In addition, there has been proposed that oilless fixation can be achieved by specifying the elastic modulus of toner containing release agent (see Patent Document 4 and Patent Document 5). Each of those documents does describe that the compatibility between OHP transparency and hot offset resistance certainly can be achieved by specifying viscoelasticity near a fixation set temperature of 150° C. or 170° C.
Furthermore, there has been disclosed that it can achieve the compatibility between low-temperature fixing ability and offset resistance can be achieved by specifying viscoelasticity in each of two temperature regions which are the temperature region of 60 to 80° C. and the temperature region of 130 to 190° C. (see Patent Document 6, Patent Document 7, and Patent Document 8).
Furthermore, there has been disclosed that the compatibility between an additional improvement in fixing ability and developing ability can be achieved by specifying the local maximal value and local minimal value of a loss tangent (tan δ) as a ratio between a storage elastic modulus (G′) and a loss elastic modulus (G″) for the viscoelastic property of toner (see, for example, Patent Document 9, Patent Document 10, and Patent Document 11).
However, problems still remain regarding how to provide developing ability that is stable for a long time period upon temperature increase in a developing device due to continuous paper transport while maintaining good fixing ability.    [Patent Document 1] JP 3-50559 A    [Patent Document 2] JP 2-79860 A    [Patent Document 3] JP 1-109359 A    [Patent Document 4] JP 6-59502 A    [Patent Document 5] JP 8-54750 A    [Patent Document 6] JP 9-34163 A    [Patent Document 7] JP 2002-13196 A    [Patent Document 8] JP 2004-333968 A    [Patent Document 9] JP 2004-151638 A    [Patent Document 10] JP 2004-157342 A    [Patent Document 11] JP 2004-264484 A